A way of grouping customers by something they share (usually signup month) and watching how that group behaves over time, so you can tell real product improvement apart from a shifting mix of users.
Andrew Chen published 'Why Investors Don't Fund Dating' in 2012, using cohort retention curves to explain why most consumer apps look successful in aggregate while leaking users behind the scenes. The essay turned the cohort table from a niche analytics view into a default slide in every consumer-app board deck.